Inside the booming field of animal law, in which animals have their own interests -- and their own lawyers.

IN RECENT YEARS, Dr. Amy Marder, a veterinarian practicing in Lexington, has found herself called upon to decide which human "parent" a pet prefers.

Pet custody disputes have become an increasingly common fixture in divorce cases and Marder, an animal behavior specialist, has consulted in several. To do a proper evaluation, she likes to spend at least an hour and a half with the couple and the pet. She asks the owners a barrage of questions: which of the two spends more time with the animal, who plays with it more, who feeds it. She asks about the pet's upbringing, its temperament, how much it exercises.

Marder frowns on so-called "calling contests," a method used by lawyers in some custody cases, in which the owners stand at opposite ends of a room and call the pet to see which way it will go. She prefers to observe the animal's body language as it interacts with its owners. She looks at whether it sits closer to one or the other, and how it reacts when each pets it.

At the end of the session, Marder makes her recommendation, based not only on who she thinks would take better care of the pet, but whom she has decided the pet has a stronger bond with - the same sort of considerations that would go into deciding a child-custody case. Sometimes she recommends joint custody, but only if she thinks the animal can handle it.

"Some animals think it's terrific to go live in two homes," she says. "Others have separation anxiety and splitting time would only make it worse."

A decade ago, the idea that a divorce would involve "custody" of a pet, much less that the decision would factor in the pet's own predilections, would have been dismissed by most lawyers as absurd. Pets were property, and not very valuable property at that, to be balanced against all of the other stuff that is split up in a divorce - nobody, after all, talks about joint custody of an armoire.

But recent years have seen an intensifying effort on the part of animal rights activists, legislators, prosecutors, and legal scholars to change the way the law treats animals.

The result has been the beginning of a qualitative shift - not merely the stiffening of animal cruelty laws, though in most states that has happened, but changes that are turning animals into legal beings with their own interests, and, in a few cases, their own enforceable preferences. Thirty-nine states and the District of Columbia now allow pet owners to endow pet trusts, the kind of legislation that made it possible for New York hotel billionaire Leona Helmsley to bequeath $12 million to her dog, Trouble. In some states, veterinarians are now required to report suspicions of animal abuse in the same way pediatricians have to report child abuse. Courts are starting to take seriously the claim that pet owners are entitled to compensation for pain and suffering in cases involving the death of an animal. And, in a Tennessee case this spring, a court appointed a legal guardian to represent the interests of a dog in a custody dispute.

These new laws and decisions have the potential to redefine the age-old legal boundary between people and their property.

"For literally thousands of years animals have been part of personal property," says David Favre, a law professor and animal law specialist at Michigan State University, "but in the past five years we're seeing courts take a broader view that animals are not like televisions and computers, that our relationship with them is more complex than that."

At the same time, the field of animal law is growing. Nearly half of the 190 accredited law schools in the United States now offer animal law courses, up from a handful 10 years ago, and around 100 now have chapters of the Student Animal Legal Defense Fund. A rising number of lawyers are dedicating themselves, in whole or in part, to the practice, and the American Bar Association and 13 state bar associations now have animal law committees.

For the most part, the lawyers arguing these new sorts of cases avoid the language of animal rights. In the eyes of the law, only people have rights, and even many animal lawyers are unwilling to dissolve the boundary between animal and person. Instead, many argue that animals should be something intermediate, a form of sentient property.

Still, a few animal lawyers see the evolution in the law paving the way to a more fundamental rethinking of the legal status of what they call, to emphasize our own connection to the animal world, "nonhuman animals."

Steven Wise, a Boston-based animal rights lawyer and a leading animal rights theorist, shares that view. "The idea that nonhuman animals are worthy of anything - that they have some value that's worthy of fighting about in court - that will lay the foundation for litigation that would actually lead to nonhuman animals getting some sort of equal rights," he says.

The belief that animals deserve their day in court is not new. In medieval Europe, animals were often held criminally accountable for their actions, in trials complete with defense counsel and character witnesses. According to E.P. Evans's 1906 book, "The Criminal Prosecution and Capital Punishment of Animals," still the definitive work on the topic, domestic animals were regularly tried for murder, assault, and even, curiously enough, "bestiality." Pigs were a particular menace, and often publicly hanged - in one case two herds of pigs were condemned as accessories to murder for having egged on three sows that attacked a young boy. In 1545, in a case that dragged on for several years, the residents of a small French wine-making town brought suit against an infestation of weevils.

Animal cruelty laws have a more recent provenance. The first in the world, passed in Massachusetts Bay Colony in 1641, forbade "Tyranny or Cruelty towards any brute creature which [is] usually kept for man's use," and mandated adequate food and water for such animals. In the agrarian pre-20th-century United States, animal cruelty statutes were understood to apply primarily to work and farm animals.

For pets - or "companion animals," the term animal rights activists and most animal lawyers prefer - protections have strengthened considerably in recent years. In the past decade, 42 states have passed felony animal cruelty laws, and in most states it's now possible to serve time in prison solely on an animal cruelty conviction (though most states now exempt farm animals from animal cruelty laws). And in the wake of the dogfighting case of NFL star Michael Vick, three bills were introduced into Congress to strengthen federal anti-dogfighting prohibitions (all 50 states already have laws of their own against it). A few district attorney offices, including Los Angeles County and Arizona's Maricopa County, have gone so far as to set up special task forces dedicated to such cases. And some prosecutors are pushing for a nationwide animal abuser database modeled on the sex offender registries most states maintain.

Law enforcement officials and prosecutors describe their concern about animal cruelty in terms that are partly instrumental. They point to research by psychologists like Frank R. Ascione, of Utah State University, and Randall Lockwood, of the American Society for the Prevention of Cruelty to Animals, suggesting that people who hurt animals are prone to violence toward people, as well.

"Regardless of how you feel about animals, some of these people are so dangerous that you want to know if they're living in your neighborhood," says Diane Balkin, a deputy district attorney in Denver who handles animal cruelty cases. "If they do these sorts of things to an animal, what will they do to children?"

Some of the changes in criminal law, however, have little to do with human protection, and consist solely of extending to animals safeguards previously reserved for people. Nine states, including Connecticut, Maine, and Vermont, have passed laws in the past two years allowing animals to be protected by restraining orders. In Massachusetts, a similar law has been introduced into the House of Representatives.

Outside the criminal realm, the shift has, if anything, been more dramatic. Estate and divorce lawyers claim to have seen a rise in the number of pet trusts and pet custody disputes. In both, animals are being discussed in legal terms that were previously reserved for children. Trouble, the late Leona Helmsley's temperamental Maltese, won't have a say in how her $12 million is spent - her trustee will - but the trustee will be acting in Trouble's best interests, just as he would for a human heir.

The rise in animal law cases, and the changes that are resulting from them, have sparked a discussion beyond legal circles over whether altering the legal status of animals lays the groundwork for giving them the sort of rights that humans have traditionally reserved for themselves.

The growth of animal law has mirrored the growing mainstream acceptance of the animal rights movement - in membership and resources, both the Humane Society and PETA have grown dramatically in the past decade, and their influence is felt not only in the strengthening of state animal cruelty laws and the vehemence of the public condemnation of Vick, but the fact that even fast food chains like McDonald's and Burger King have felt compelled to address the conditions of the cows and chickens they rely on.

Not everyone, though, is enthusiastic about the prospect that animal law might be put to the service of animal rights. Veterinarians tend to be particularly leery of expanding the legal claims of animals - the fact that courts have started awarding non-economic damages in veterinary malpractice cases, vets warn, will only raise the overall cost of care.

In many states, such damages apply only in the death of a spouse or parent or child, not a best friend, say, or a fiance. According to Adrian Hochstadt, assistant director of state legislative and regulatory affairs for the American Veterinary Medical Association, allowing pet-related non-economic damages rewards shows that the nation is headed down a slippery slope.

Yet for some legal scholars, like David Favre and Steven Wise, the slippery slope is exactly the point. The current changes in how we protect, provide for, and fight over animals, they argue, are a precursor for urgently needed, and more fundamental, changes in the law, especially concerning highly intelligent animals such as chimpanzees and dolphins. Favre points to several models he'd like to see American animal law follow: the German constitution, which in 2002 gave constitutional rights to animals; New Zealand, which has recognized limited personhood for primates; and the European Parliament, which recently categorized great apes as "beings" and moved to end the use of all primates in scientific research.

Other animal lawyers, though, see something less sweeping: a legal system trying to capture the evolving but still deeply ambivalent feelings most people have about animals. In this model the law isn't driving change, but playing catch-up.

"I am one that believes courts are always behind society," says Jonathan Rankin, a lawyer who recently left the Boston firm Glickman Turley to start his own animal law practice. "If corporations can be persons in the eyes of law, if ships can be persons in the eyes of the law, then the law should be able to figure out something for animals."